The invention relates to a device for detecting an overlapping edge of material on a sewing machine and, more particularly, for detecting the edge of one layer of material, such as a patch pocket, overlapping another, larger layer of material, such as a garment, to stop an industrial sewing machine sewing the layers together at the edge.
When, as is frequently desired, a smaller piece of material, such as a patch pocket, for example, is to be sewn onto a larger piece of material, such as a garment, for example, it is necessary to stop at least the stitching mechanism and, generally, also the feeding mechanism of a sewing machine just before the edge of the smaller piece of material on the larger piece of material is reached to form a commercially-acceptable seam between the pieces. The high speed of industrial sewing machines makes this exacting and the complex and compact constructions of the sewing heads and beds of such machines makes it difficult to arrange sufficiently exacting devices at the sewing heads or beds, where required.
German patent publication OS No. 26 52 261 discloses a device for controlling an industrial sewing machine which detects obstructions in the material to be sewn, such as folds or the like, with a mechanical scanner contacting the material to be sewn. When the mechanical scanner detects an obstruction, it initiates a reaction according to a predetermined program in the driving (stitching and feeding) mechanism of the sewing machine. This device cannot detect an overlapping edge of a smaller piece of material being sewn onto a larger piece of material, however, because it is not an obstruction.
Devices having an emitter and a detector in positions for the detector to receive a reflection of radiation emitted by the emitter, generally on the sewing head above material being sewn on a sewing machine, are also known for controlling a sewing machine. These devices detect the edge of a piece of material on a support having a coefficient of reflection different from the material. This situation generally does not exist at an edge of one piece of material which overlaps another piece of material. The devices, are, therefore, not suitable for controlling a machine for sewing these together.
German Patent publication No. 33 23 214 discloses a device for controlling a sewing machine with transmitted light. It has two electro-optic sensors tandem along in the sewing direction and circuitry for scanning the signals of the sensors and supplying them to a frequency converter, a rectifier and, finally, a discriminator. In this manner, it is possible to detect the intensity difference which arises when one sensor is irradiated through one layer of material and the other sensor, through two layers of material, i.e., when an overlapping edge of one of the materials comes into the region between the two sensors. The circuitry for this is, however, very expensive and it is necessary to arrange both of the two sensors with appropriate connecting leads in the immediate vicinity of the sewing.